This invention is related to electromagnetic pumps and more particularly to direct electromagnetic pumps having pole extensions for improving the efficiency of the pump.
As is well known in the art, electromagnetic pumps produce a pressure differential or a pressure head between the inlet and the outlet through the interaction of an electrical current and a crossed magnetic field. This interaction produces an electromagnetic force density throughout the volume of the fluid within the pump region wherever both the current density and the magnetic field are non-zero. At each such point, this force is proportional not only to the magnitude of the current density and magnetic field, but also to their relative orientation. The maximum force density and resulting pressure differential occurs when the electrical current and the magnetic field are mutually perpendicular to each other and to the direction of fluid flow.
Typically, the electromagnetic pumps are constructed in a rectangular duct by mounting two electrodes flush with the opposite side walls of the duct and placing the other two walls between magnetic pole faces. When the two electrodes are connected to an external power supply, current flows across the duct and interacts with the magnetic field to produce the axially directed body force and pressure difference across the duct. The pump's inlet and exit regions are defined roughly by the electrode edges. These regions may vary somewhat depending upon the relative location of the magnetic pole face edges. In an ideal pump, all the current would be confined to the duct volume enclosed by the electrodes and the pole faces where the force density is the greatest. In an actual pump, however, some current leaks into the magnetic fringe region both upstream and downstream from the electrode edges. This tends to lower pump efficiency. Thus, current leakage in the magnetic fringe regions adds little to the overall pressure differential while increasing the current flow thereby diminishing efficiency.
One technique often discussed in the literature for improving efficiency is to extend or "grade" the magnetic field decay such that a gradually diminishing back emf is established that tends to oppose leakage current. In should be noted that, simply extending the magnetic pole faces in order to extend the region of high field intensity beyond the electrode edges does not necessarily improve efficiency. Simply extending the high interpole field region, in fact, may be counter productive since the increased field beyond the electrodes may create a back emf greater than the applied electrical potential thereby producing reverse circulating currents and a negative pressure gradient in the duct. A properly graded field, however, may reduce leakage currents while at the same time extracting a positive pressure contribution from the fringe regions.
In addition to the electromagnetic loss effects, viscous effects also contribute to power losses. It has been suggested that viscous effects coupled with electromagnetic body forces produce velocity profiles in the duct characterized by large axial velocities or jets near the electrodes that quickly diminish to a gradually sloped profile at positions away from the electrode walls and closer to the center line of the duct. In some circumstances, it is believed that the flow of the fluid may stagnate or even reverse along the center line of the duct under these conditions. Viscous-drag losses along the duct walls and the increased turbulence and complicated back flow along the duct center line lead to increased viscous dissipation and lower pump efficiency. Therefore, what is needed is an improved electromagnetic pump wherein electrical current leakage and viscous dissipation are reduced, thereby increasing pump efficiency.